1. Technical Field of the Invention
The present invention relates generally to a spatial light modulator and a lighting system therefor, and more particularly to improved structures of a spatial light modulator and a lighting system for producing a contrasty modulated light in projection displays, projectors, and optical information processing devices.
2. Background of Related Art
Nowadays, projection displays using a liquid-crystal panel as a spatial light modulator are being developed. Particularly, projection displays using a reflective liquid-crystal panel are attractive since the aperture ratio of transmission liquid-crystal panels is 50%, while that of the reflective liquid-crystal panels may be increased more than 90%. Projection displays of this type are already put into practical use for a specified purpose.
FIG. 1 shows a lighting system and a spatial light modulator of a conventional projection display using a reflective liquid-crystal panel.
The spatial light modulator 1 is made of a lamination of the reflective liquid-crystal panel 5, the glass plate 6, and the coupling prism 7.
The liquid-crystal panel 5 has a structure in which MOS transistors and A1-made reflective mirrors (i.e., pixel electrodes) are formed on an Si substrate on a pixel basis, and a liquid-crystal layer is formed between the Si substrate and a transparent common electrode. Modulation and reflection of readout light incident on the liquid-crystal layer in units of pixel are achieved by activating a portion of the liquid-crystal layer between one of the reflective mirrors and the common electrode through an active matrix system.
The MOS transistors and their connection wires may be disposed below the reflective mirrors to use almost the entire surface of the panel as a reflective surface regardless of size of the MOS transistors, thereby achieving a great aperture ratio, as discussed above, even if the pixels are increased greatly.
The lighting system includes the polarizing plate 2, the integrator 3, and the light source 4 such as a tungsten lamp or a metal halide lamp.
The unpolarized light is emitted from the light source 4 and enters the polarizing plate 2 through the integrator 3 wherein only a component of the unpolarized light polarized in one direction (i.e., an s-polarized component) is selectively allowed to pass therethrough and falls on the spatial light modulator 1. The optical axis of a luminous flux of the unpolarized light incident on the polarizing plate 2 is oriented at a given angle .theta. to a normal at any point on the liquid-crystal layer of the reflective liquid-crystal panel 5 so that the s-polarized component (i.e., readout light) emerging from the polarizing plate 2 diagonally enters (i.e., an incident angle=.theta.) the liquid-crystal layer through the coupling prism 7 and the glass plate 6.
The entrance surface 7a of the coupling prism 7 is oriented perpendicular to the optical axis of the unpolarized light, and the coupling prism 7 and the glass plate 6 are made of glass materials having the same index of refraction, so that the readout light enters the liquid-crystal layer directly without undergoing refraction.
Although not illustrated in the drawing, a cold mirror may be interposed between the integrator 3 and the polarizing plate 2 for removing components within a waveband of infrared light. Further, three-plate projection displays designed to produce full color images may separate the incident light into R, G, and B rays and transmit them to corresponding liquid-crystal panels, respectively. Optical paths defined between the projection display and the liquid-crystal panels are commonly curved.
The s-polarized component incident on the liquid-crystal layer is reflected and returned by the reflective mirror to the liquid-crystal layer and undergoes modulation by control of transition of liquid-crystal molecules through activities of the reflective liquid-crystal panel 5 in response to a pixel signal, so that only a p-polarized component (i.e., modulated light) whose plane of polarization is rotated on a pixel basis is outputted from the glass plate 6 to the coupling prism 7 and emerges from an exit surface thereof, which is, in turn, directed to a projection optical system (not shown) so that an image is projected onto a screen.
In the above described projection display, the polarizing plate 2, as can be seen in the drawing, has a polarized light selecting plane oriented perpendicular to the optical axis of a luminous flux of incident light. Specifically, the polarizing plate 2 has the transmission axis oriented perpendicular to the drawing of FIG. 1 to allow a component of incident light parallel to the transmission axis to pass therethrough.
The light source 4 is not an ideal point source of light, but a source of radiation having a limited extension in space. The integrator 3 includes a first lens plate 3a and a second lens plate 3b. The first lens plate 3a has fine lens segments disposed thereon in a matrix arrangement which produce a plurality of secondary sources of light. Each of the lens segments of the second lens plate 3b converges the incident light on a flat area (i.e., a readout light-illuminated area) of the liquid-crystal layer of the reflective liquid-crystal panel 5. Specifically, a collection of conical fluxes of light is incident on the polarizing plate 2. Of each of the conical fluxes, other than light beams perpendicular to the entrance surface of the polarizing plate 2 enters the polarized light selecting plane of the polarizing plate 2 at angles other than 90.degree..
FIG. 2 shows a polarized light selecting function performed by the polarizing plate 2.
The polarized light selecting plane of the polarizing plate 2 serves to select a component of incident light to be polarized on a crossed line defined by a plane that is perpendicular to the travel direction of the light and a plane that contains the transmission axis, as indicated by a white arrow, of the polarizing plate 2 and is perpendicular to the polarized light selecting plane and allows the selected component to pass therethrough.
Thus, of the light beam (A) incident at right angles to the polarized light selecting plane of the polarizing plate 2, a component whose direction of polarization is identical with the transmission axis of the polarizing plate 2 (i.e., the s-polarized component [S]) is selected and allowed to pass through the polarizing plate 2. Of the light beam (B) incident diagonally on the polarized light selecting plane of the polarizing plate 2, a component whose direction of polarization is on a plane that contains the transmission axis and is perpendicular to the polarized light selecting plane (i.e., the s-polarized component [S']) is selected and allowed to pass through the polarizing plate 2.
The s-polarized components [S] and [S'] fall on the readout light-illuminated region 5a of the reflective liquid-crystal panel 5. The direction of polarization of the s-polarized component [S] is parallel to the surface of the region 5a, while the direction of polarization of the s-polarized component [S'] is not parallel to the surface of the region 5a. Specifically, the direction of polarization of each light beam incident on the polarizing plate 2 depends upon an incident angle thereof. The region 5a is, thus, illuminated by light beams whose directions of polarization are different from each other, which will cause the light beams incident on the reflective liquid-crystal panel 5 to undergo different modulations, thus resulting in reduction in contrast of an image formed by the modulated lights with a screen.